In many polymer manufacturing and processing operations it is necessary to transform a polymer solution or a polymer melt into solid polymer pellets or crumbs for packaging in sacks or other containers or bales. Such processes are generally referred to as polymer finishing and packaging and the associated apparatus may be referred to as a polymer finishing and packaging line. Thus, in polymer manufacture, the polymer may be produced directly as a polymer melt or alternatively may be produced as particles in a slurry polymerization which are then melted to form a polymer melt or as a polymer solution in a solvent from which the solvent is evaporated away while the polymer is left behind either as crumbs or as a melt which is then subject to finishing and packaging operations. In polymer processing operations of similar need arises, for example, where two or more different polymers have been blended or are being combined with non-polymer components such as a colorant or filler. One process of interest which produces polymer as a melt is the continuous solution polymerization of olefins.
Continuous solution polymerization processes generally involve the addition of catalyst to a monomer and solvent mixture. The mixture may be back-mixed giving a uniform polymer in an environment with substantially no concentration gradients. WO 94/00500 (Pannell et al.), incorporated herein by reference, describes a solution polymerization using metallocene in a continuous stirred tank reactor or multiple reactors which may be in a series or a parallel reactor arrangement to make a variety of products.
The heat of the polymerization reaction can be absorbed by the polymerization mixture, causing an exotherm. Alternatively, or in addition, the heat of reaction can be removed by a cooling system, by external cooling of the walls of the reactor vessel, or by internally arranged heat exchange surfaces cooled by a heat exchange fluid, or by circulating the reactants through an external heat exchanger, or by allowing some of the solvent to evaporate.
In the course of the polymerization, typically, a predominant amount (over 50 mol %) of the monomer and/or co-monomers is consumed and the polymer formed is dissolved in the solvent. The higher the concentration of the polymer, the higher the viscosity of the polymerization reaction mixture containing the polymer, solvent, and unreacted components. The mixture passes from the polymerization reactor to a finishing section in which polymer, solvent and unreacted monomers are separated. In the course of finishing, solvent and unreacted monomer are progressively removed from the polymerization mixture until the polymer can be formed into solid pellets or bales ready for packaging. The pellets or bales are then conveyed to a packaging line for packaging, for example in sacks or large or other suitable containers. The separated solvent and monomer can be recycled to the polymerization reactor.
The finishing section may also comprise a vacuum devolatilizer, in which the molten polymer is exposed to a vacuum while being intensively agitated to draw off volatiles such as solvent and residual monomer, in order to reduce the level of volatiles in the finished polymer to a desired level.
U.S. Pat. Nos. 6,881,800 and 7,163,989, both of which are incorporated herein by reference, describe a process and apparatus for the continuous solution polymerization of olefins including ethylene, propylene and other olefin comonomers. The polymerization reaction takes place under pressure in one or more polymerization reactors, and then the effluent from the reactor or reactors is treated in a finishing section with a catalyst killer and then heated in one or more heat exchangers before being subject to a pressure drop which causes the effluent to phase separate into a polymer-rich phase and a polymer-lean phase. Those phases are separated, with the polymer-lean phase being purified and recycled to be used as solvent. The polymer-rich phase is subject to further separation and purification stages, including passage through a vacuum devolatilizer. Following the vacuum devolatilization, the polymer is formed into pellets and/or bales for storage or shipping. The process is suitable for the manufacture of a range of different polymer types.
In some solution processes (see WO 98/02471 Kolthammer), incorporated herein by reference, the polymerized mixture is flashed off in two stages, whereby the solvent and unreacted monomer are converted to a vapor phase. Efficient extraction of solvent, etc., requires low vapor pressures and vapor phase compression or condensation followed by pumping for subsequent separation stages. Pumping is used to convey polymer from flash separation stages to a final devolatilizing extruder.
In solution plants, solvent selection, operating temperatures, and purification systems have to be designed for a particular operating window for the desired polymerization process. Metallocene catalysts permit a wide variety of polymers to be made in terms of comonomer content, molecular weight, etc. Optimum production performance for a given type of polymer may be obtained with a particular catalyst within a specific operating window. Different types of polymer may then have to be produced in different plant lay-outs. There is, therefore, a need for a plant design that can be used more flexibly for different types of polymers and catalysts, and which also can be adapted more easily to evolving catalyst technologies than current designs of solution polymerization plants.
Some polymers produced using solution polymerization are soft and tacky, and are therefore prone to handling problems. There is therefore a need for plants and processes which allow improved handling of such soft polymers.
Some polymers which are generally not subject to handling problems caused by tackiness are nonetheless slow to crystallize from the melt, and are therefore subject to handling problems until crystallization has reached a significant level. There is a need for improved plant and processes for handling such slow-crystallizing polymers.
Some polymers are prone to oxidation by atmospheric oxygen, giving rise to gels and other imperfections in the finished product. There is also a need to reduce such gels and other contaminants.
Polymer manufacturing and processing plants are often located in regions having hot and humid climates, and can suffer from packaging problems caused by the high humidity. There is a need for plant and processes which offer an improved way of handling polymer in humid climates.
For additional background, see also WO 94/00500 and WO 92/14766, both of which are incorporated herein by reference.